fix(workouts): one series, one enrichment, when a batch names a workout twice

Two entries carrying the same external id and both a `samples` array
reached the enrichment arm one after the other, and both read the same
landed workout id back, so the response claimed two attaches where the
database had written one. The second entry now takes the duplicate arm:
a workout is claimed by the first entry of the batch that reaches it.

tests/integration/workout-batch-enrich.test.ts

@@ -213,6 +213,33 @@ describe("POST /api/workouts/batch — HR series enrichment (real Postgres)", ()
     ).toBe(1);
   });
 
+  it("reports one enrichment when two entries in one batch aim at the same workout", async () => {
+    await post({ workouts: [baseWorkout("hk-uuid-enrich-007")] });
+
+    // Same external id twice, far enough apart in time that the
+    // write-time twin dedup keeps both entries. Only one series can land.
+    const res = await post({
+      workouts: [
+        baseWorkout("hk-uuid-enrich-007", { samples: SERIES }),
+        baseWorkout("hk-uuid-enrich-007", {
+          startedAt: "2026-05-14T09:00:00.000Z",
+          endedAt: "2026-05-14T09:45:00.000Z",
+          samples: [{ t: "2026-05-14T09:00:00.000Z", hr: 155 }],
+        }),
+      ],
+    });
+
+    expect(res.status).toBe(200);
+    expect(
+      res.data.entries.filter((e) => e.status === "enriched"),
+    ).toHaveLength(1);
+    expect(
+      await getPrismaClient().workoutSamples.count({
+        where: { workout: { userId: TEST_USER_ID } },
+      }),
+    ).toBe(1);
+  });
+
   it("leaves every workout column byte-identical across an enrichment", async () => {
     await post({
       workouts: [